A review on chromosome mutations formed in different phases of the cell cycle permits conclusions on the fate of newly induced aberrations. Chromatid-type aberrations expected by the action of chemical mutagens are likely to be soon eliminated from a cell population. Different mutagens may show different effects. These considerations are confirmed by investigations of mitoses of bone marrow cells and spermatogonia. The bone marrow of rodents proves to be a suitable test system for potential chemical mutagens. Observations of the meiotic system do not allow conclusions regarding the mutagenicity of a substance to be tested as may be judged from our present knowledge.